GARNET GEOCHRONOLOGY AND PETROLOGY OF THE OSGOOD SCHIST: IMPLICATIONS FOR GRAPHITIZATION

Graphite-ore is hosted in calc-silicate bearing mica schist of the historic Osgood Mine (Read, 1892), and regionally throughout the sillimanite-grade metasedimentary rocks of New Hampshire (Rumble III et al., 1986). Graphite occurs as rectangular inclusions, disseminated flakes, and massive clusters in fracture fillings within garnet porphyroblasts, as well as scattered throughout the matrix. The purpose of this project is to constrain the timing and mechanisms of graphitization using textural and chemical analysis in addition to garnet geochronology. New Sm-Nd garnet geochronology indicates that metamorphism occurred at 406.3 ± 1.7 Ma. This is concurrent with 400 – 411 ± 10 Ma igneous ages (Lyons and Livingston, 1977, Chamberlain and England, 1985, Pyle et al., 2005), and implies metamorphism occurred during the Acadian Orogeny. The presence of two distinct textural generations of graphite within garnet: 1) as inclusions and 2) within garnet fracture fillings, indicates that graphitization occurred in two events, one before and one after garnet growth. Scanning electron microscope (SEM) compositional analysis revealed rims of pyrite on hematite encasing matrix graphite. X-Ray fluorescence (XRF) analysis indicates major losses of silica and alkali elements (K₂O and Na₂O), as well as bulk enrichment in titanium and total iron. Chemical analyses suggest significant metasomatism based on micron-scale interactions between iron-bearing phases and the bulk depletion of readily soluble elements (silica and alkalis) and enrichment in low-solubility elements (titanium and total iron) (Ague, 1991). Graphite emplacement at the Osgood Mine occurred in two stages, initially during garnet growth and later during a period of intense hydrothermal alteration accommodated by intersecting fracture networks, retrograde metamorphism, and a strongly reducing fluid phase rich in carbon-bearing volatiles.